arm/arm64: ITS: Introspection tests

Detect the presence of an ITS as part of the GICv3 init
routine, initialize its base address and read few registers
the IIDR, the TYPER to store its dimensioning parameters.
Parse the BASER registers. As part of the init sequence we
also init all the requested tables.

This is our first ITS test, belonging to a new "its" group.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>